I get a byte array from ssrs report. Then I want to save it in Excel on server for further processing.
I am able to export it on client browser but when try to save the file on server it saves. But gives the error while opening "excel can not open the file because the file format or file extension is not valid".

@Jodrell in msdn link it convert it to .xls format. Can it be converted into .xlsx format.
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rahulOct 11 '13 at 9:50

What version of SSRS are you using? If you have 2012+ then you'll have compatability problems with Excel 2003. Otherwise, 2007+ versions of Excel should be compatible with legacy Excel files.
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JodrellOct 11 '13 at 10:13